Gas turbine engines are complex and finely tuned machines. Generally, such an engine is composed of a low pressure compressor and an associated low pressure turbine, the speed of the low pressure turbine is typically designated N1. The gas turbine engine also includes a high pressure compressor and associated high pressure turbine, the speed of which is typically designated N2.
Accurate knowledge of N2 is an important factor to maintaining efficient and high performance in gas turbine engines. However, N2 speed is difficult to measure directly. The most common indirect measures involve measuring various engine air pressures and interpolating N2. This process can produce wide variations in results, particularly over time due to engine wear.